In the microelectronics industry, surfaces that are typically scratch-free are polished for the purpose of planarizing the structure involved and/or removing unwanted material. By way of example, metals such aluminum, copper and tungsten are planarized. These metal surfaces are oxidized so that the polishing abrasive does not produce scratches. Moreover, there is typically a refractory metal liner underneath the aluminum, copper or tungsten, providing good adhesion to the underlying insulator and good contact resistance to the lower level metallizations. The liners can be niobium, tantalum and titanium alone or in combination with their nitrides, or any other refractory metal.
In practice, it is often necessary that a two-step procedure be used to achieve the desired polishing results. A primary polish is used to remove large amounts of the primary material and then a secondary polish for removing a different underlying or liner material. By way of example, the slurries typically contain an abrasive particle such as alumina, silica, ceria, zirconia, or titanium dioxide, along with an oxidizing agent such as ferric nitrate, potassium iodate, ammonium cerium nitrate, potassium ferricyanide, silver nitrate, sodium hypochlorite, potassium perchlorate, potassium permangenate or hydrogen peroxide. The primary and secondary slurries differ both in the kind of abrasive applied and in their chemistry. For instance for metal polish, typically the primary slurry applies alumina abrasive and has an acidic pH, while the secondary slurry applies silica abrasive and has a more neutral pH.
After polishing, it is necessary to clean the polished surface to remove the polishing slurry and polishing debris with a minimum of chemical or mechanical surface damage.
A widely used wafer cleaning method involves mechanical removal whereby the wafers after polishing are passed through one or two pairs of brush-cleaners that are wetted with deionized water. This technique, however, leaves the wafers and the brushes with a relatively high content of polishing debris particles referred to as particle count. This results in reducing the wafer yield and/or limiting the brush life, which is a relatively expensive item.